Mobile speech communication is handled mainly via cell-based mobile radio networks. In Europe, cell-based mobile radio networks such as these are based on the GSM standard or the UMTS standard. The use of these mobile radio networks is costly for the user, with charges being incurred on the basis of the call duration.
The main field of application of wireless local area networks, also referred to as WLANs, is, in contrast to this, non-speech-oriented data transmission. In this case, WLAN forms a generic term for a large number of different standards for local wireless networks. These include in particular the various standards from the IEEE-802.11 Working Group (for example IEEE-802.11b or IEEE-802.11e), the Bluetooth Standard, the HomeRF Standard and the HiperLAN Standard.
So-called hot spots are being installed at public locations owing to the increasingly widespread use of WLANs such as these, in particular to the IEEE-802.11b standard. A so-called hot spot is produced within radio range around an IEEE-802.11-compatible WLAN base station, also referred to as a WLAN access point, in which case, a radio link can be set up within the hot spot between a WLAN-compatible mobile appliance, for example a laptop with a WLAN radio network card, and the WLAN access point. The WLAN access point is also connected to the landline network, for example via a wire-based Ethernet interface. The Internet can be accessed via this wire-based interface. Thus, when within radio range of the WLAN access point, the user of a WLAN-compatible mobile appliance can thus access the Internet via the WLAN access point. In some cases, the use of this infrastructure costs nothing, or is billed on the basis of the amount of data, the access duration or on a flat-rate basis.
Internet telephony, also referred to as VoIP (Voice over Internet Protocol) can be operated via a WLAN access point by access to the Internet. In this case, the speech information is interchanged via the Internet on a packet-oriented basis. VoIP places a number of requirements on the transmission channel, that is to say both on the radio link between the WLAN base station and the WLAN access point and on the wire-based link between the WLAN access point and the communication partner. These requirements and the compliance with these requirements are also referred to as Quality of Service (QoS). One particular requirement in this case is that it is possible to guarantee a data rate which is adequate for transmission of the speech information. A further requirement is for the delay time on the transmission channel to remain constant in time, so that the transmitted signal has little time jitter. As a further essential requirement for the transmission channel, it is necessary to ensure that the delay time on the channel remains below a threshold which is still acceptable for speech communication. In this case, in particular, it is necessary to take account of the fact that the bidirectional delay time, which is normally also referred to as the “round trip delay” (RTD), between the transmission of an information item via the transmission channel and the reception of an information item based on the transmitted information should at most be in the range of less than 100 ms.
If the transmission channel does not satisfy the abovementioned requirements, then a VoIP application is associated with major quality restrictions. If the requirements of the transmission channel can be complied with, it is possible to replace speech communication via a cell-based mobile radio network by VoIP. Speech communication in the radio area of a WLAN access point can in this case be carried out at considerably less cost, or even without any cost, via WLAN instead of via a cell-based mobile radio network. This results in a competition situation between IEEE-802.11-based WLAN radio networks and the cell-based mobile radio network. Since the operators of cell-based mobile radio networks also operate WLAN hot spots, this competition situation even occurs operator-internally. Owing to the cost advantage of a VoIP call over an IEEE-802.11-based WLAN radio link in comparison to a call via a GSM or UMTS mobile radio appliance, there is a risk of the turnover for cell-based mobile radio being reduced.
The IEEE-802.11e Standard, which has not yet been finally specified, will provide novel access methods which allow an improved QoS, in particular for VoIP applications, by appropriate prioritization of the data traffic. In this case, it will be possible for the improved QoS to be made available only to appropriately authorized users, in order to limit the VoIP traffic. Since, however, the improved QoS is not absolutely essential for VoIP applications when the load level on the WLAN access point is at a low or medium level, the VoIP traffic cannot be restricted or monitored by selective provision of an improved QoS.
In principle, the WLAN access point cannot tell whether the data traffic on a radio link between a WLAN mobile station and a WLAN access point is governed by VoIP-based data or non-speech-based data. It is therefore not possible for the WLAN access point to block the WLAN radio link for unauthorized VoIP-based data.